Coprocessors: Add metrics for custom RPC methods called through HTable.coprocessorExec()

Details

Description

Currently HBaseRpcMetrics only reports on known RPC methods in core HBase. With HBASE-3405 we added hooks to register metrics for new methods. We can make use of these for tabulating metrics on custom CoprocessorProtocol extensions invoked through HTable.coprocessorProxy() and HTable.coprocessorExec().